It is known to provide switching systems with duplicated facilities operated on an on-line/off-line basis. This is done to increase system reliability. The off-line unit is always available as a spare in case the on-line unit malfunctions.
It is common practice in duplicated systems to "exercise" the off-line unit by periodically switching the on-line/off-line status of the duplicated facilities as a preventative maintenance measure. This ensures that an operable spare is always available. The status switching of the duplicated units is usually performed during periods of light traffic, such as at 2:00 a.m. This is done to minimize any possible service disturbances or interruptions that might be encountered if the switching were done when the system is serving heavy traffic.
The switching of duplicated facilities results in little or no service degradation to "voice" traffic since, at the most, only a "click" is heard by parties to existing connections. The problem is more serious in PCM type systems serving data traffic. In these, a perturbation caused by a status switch that would cause only a "click" on a voice traffic connection could cause data to be lost or mutilated on a data connection. It is no solution to perform the status switch at 2:00 a.m. since, unlike voice traffic, data traffic can be high at any time of the day or night.
System designers attempt to overcome the above problems by synchronizing, to the extent possible, the phase and frequency of the clock signal sources in the duplicated units of PCM systems. However, it is not economically feasible to achieve the precision required in phase and frequency synchronization so that, without more, no clock signal perturbation results that could adversely affect data traffic.
Thus, it is still a problem in switching systems serving data traffic to switch the on-line/off-line status of duplicated facilities without momentarily degrading data traffic.